Presently, many types of alloys are used in the production of surgical needles Some such alloys are martensitic stainless steels, austenitic stainless steels, and plated plain carbon steel. These alloys range from materials which exhibit acceptable characteristics regarding corrosion resistance, strength and ductility. Of course, primary among all these factors is strength. Naturally, the ultimate tensile strength of an alloy is ideally as high as possible for manufacture, while not compromising any of the other characteristics of the material. The ultimate tensile strength of the precipitation grade steel can be described as a combination of an annealed strength added to a work hardening response, which is aged hardened. In general, it is desirable for current chemistries from which needles are formed to have an ultimate tensile strength greater than 400,000 pounds per square inch (400 ksi).
In general, the alloys on which this application focuses are called maraging stainless steels. This terminology indicates hardening by martensitic transformation, with precipitation hardened by aging. Stainless steel means a relatively high chromium level in the alloy, usually 12 percent or greater.
The first stage in processing these steels is annealing, or solution treatment. This entails heating the material to a suitable temperature (between 1500.degree. F. and 2100.degree. F.), sufficiently long to place one or more constituent elements into solid solution in the base metal. More preferably, maraged steels are solution treated between 1980.degree. F. and 2080.degree. F. The phase change of the solution from an austenitic state to its martensitic state commonly occurs in these alloys during cooling from the elevated temperature of the solution treatment. A rapid cooling rate insures that constituents remain in super saturated solid solution, also avoiding unwanted precipitation that might occur during a slow cool. The transformation to martensite is therefore a diffusionless phase change. Alloy additions remained trapped in solution within resulting martensite, filling interstitial sites of the base metal. In this regard, the additions block dislocation propagation and further strain the structural lattice of the alloy. Certain alloy additions may also cause martensite refinement, thus hardening or toughening the alloy due to finer martensite plate spacing.
Next, the alloy is work hardened to gain additional strength. Work hardening is a process which increases the strength of a metal by the addition of mechanical deformation. Any process that increases the resistance to slip or the motion of dislocations in the lattice structure of crystals will increase the strength of the material. In work hardening this resistance is caused by immobile obstacles generated during the deformation process itself. They can be arrays of other dislocations or grain boundaries, the number of which is also increased by the mechanical work.
Finally, precipitation or age hardening is accomplished by aging the alloy at intermediate temperatures, high enough to reactivate both diffusion and the formation of intermetallic compounds. Generally age hardening occurs between temperatures of 750.degree. F. to 1050.degree. F. Preferably maraged steels are precipitation hardened between about 825.degree. F. and 975.degree. F. A dispersion of fine precipitates nucleate at dislocations and at martensite plate boundaries, resulting in further hardening of the alloy.
Balancing ultimate tensile strength with corrosion resistance and ductility in the maraged steel is difficult to arrange. Many attempts yield high tensile strengths and yet low corrosion resistance, or low ductility. Ultimately therefore, it is the goal of this alloy to balance these criteria, and to produce a strong, ductile and corrosion resistant alloy.